The last time Megan Fox appeared on "The Tonight Show With Jay Leno" in February, the 26-year-old actress was hiding a small baby bump.

"I was about two months pregnant. I was really sick," the star of "This Is 40" said when she returned for the Dec. 18 episode of the NBC talk show. "I was very nauseous. I had really bad morning sickness."

Fox continued, "It was so bad for me. ... I was convinced that I was, like, maybe birthing a vampire baby like the one in 'Twilight' -- you know what happens to Bella, where she's in cold sweats all the time -- and I felt like that was happening because I had no vitamins and nutrients, and I was just nauseous."

Luckily, Brian Austin Green's wife of two-and-a-half years didn't suffer for long. "The first trimester was really rough, and then I got to about 12 weeks and then it was fine," the actress said. "It was really a perfect pregnancy after that."

Fox -- already stepmom to Green's son Kassius, 10, from his previous relationship with actress Vanessa Marcil -- also opened up about giving birth on Sept. 27 at Cedars-Sinai Medical Center in Los Angeles. (Coincidentally, Reese Witherspoon and Jim Toth welcomed a son, Tennessee, at the same hospital that same day.)

"I thought I was going to be tough and the nurses would be like, 'She's a warrior princess! She doesn't need an epidural! She's amazing! We're such big fans!'" Fox admitted. "As soon as I got out of the car, I was already crying for an epidural. I was asking the security guard for an epidural!"

"This Is 40" is in theaters Friday, Dec. 21. "The Tonight Show With Jay Leno" airs weeknights at 11:35 p.m. on NBC.